You do absolutely nothing. Your girlfriend is an adult. She is the only person who decides what spiritual path to follow. What gives you the right to decide what is good for her or not?
Personally I am skeptical about the word "cult" which is a broad term for "religious group I don't like". When I was a kid, I was taught that the Catholic Church and the Mormons were cults. I am especially skeptical of the word "escapee", the experiences of one person aren't the experiences of everyone.
But either way, your girlfriend is an adult. She makes decisions for her own life. That being said.
1) Like any other relationship, you either accept it or you don't. We don't get the right to mould our partners into what we want them to be, you have to accept them for who they are. If you can't accept it, then leave her.
2) I know people who left fringe religious groups on their own (without calling themselves 'escapees'). Having good relationships with people outside the group who accept you for who you are makes this easier. Accept her as who she wants to be... that is the best way to be in a position to support her if she ever decides to leave the group.